1. Field of the Invention
This invention relates to a protective means for a feeder of machinery with a DC-motor, and more particularly to an improvement of a means for protecting the armature of the DC-motor against excessive current.
2. Prior Art
Feeders for shifting a work-table of machinery are generally driven by a hydraulic cylinder mechanism. A hydraulic cylinder mechanism is advantageous in being readily controllable and able to exercise greater driving power; therefore, it is suited for a means to create thrust to shift a work-table. However, a hydraulic cylinder mechanism suffers some drawbacks such as inexact positioning of the work-table and a shorter life as well as insufficient productivity due to changes in temperature or quality of liquid to be used. In addition, it will easily cause pollution problems such as oil leakage and noisy operation.
In place of the conventional hydraulic cylinder mechanism mentioned above, it has been proposed to move the work-table by driving a motor. However, this did not enjoy all the advantages provided by the hydraulic cylinder mechanism. Particularly in the case of a AC-motor, it is not easy to change the revolution speed quickly and accurately, whereby it is needed to have a complex gear box and the like which will render the total price very expensive. In addition, because of the gear box, it is difficult to keep up the exact positioning of the work-table; therefore, the work-table must be repeatedly moved back and forth when being positioned, thereby shortening its life because of friction.
In an attempt to resolve those drawbacks mentioned above, DC-motors have been introduced to replace AC-motors, since DC-motors are advantageous in controlling the positioning of the work-table by adjusting the current in the armature, and particularly suited for compact type feeders. However, there exist a serious disadvantage in using a DC-motor for this purpose. Namely, the current tends to change too much in the armature thereby causing damage to the DC-motor due to unavoidable excessive current once in a while.